1. Field of the Invention
The present invention relates to a load cell comprising a Roberval mechanism made up of two parallel arms and of four strain-generating-portions provided in such way that two strain-generating-portions are coupled to each other on each of the two arms.
2. Prior Art
A typical load cell 1 comprising a Roberval mechanism that has been used in the prior art is illustrated in FIG. 19. The load cell 1 includes a strain-generating-body 2 and strain gauges 3a, 3b, 3c and 3d. In particular, the strain-generating-body 2 includes a fixed end 4 and a movable end 5 to act as a cantilever. The strain-generating-body 2 further includes, between the fixed end 4 and the movable end 5, an aperture 7 for strain-generating-portions 6a and 6c and an aperture 8 for strain-generating-portions 6b and 6d each for detection of strain by the strain gauges adhered thereon. A channel 9 communicating between the apertures 7 and 8 is provided so that two arms h1 and h2 are arranged in parallel to each other to form links between the strain-generating-portions 6a and 6b and between 6c and 6d. As the result, a Roberval mechanism is provided in which the four strain-generating-portions 6a, 6b, 6c and 6d are each moved as a locus of each of four corners of a parallelogram. Accordingly, a degree of strain is detected by a Wheatstone bridge circuit made up of the four strain gauges. It is assumed that the strain-generating portions 6a, 6b, 6c and 6d have the same thickness and the arms h1 and h2 have the same thickness so that the load cell is formed in vertically symmetrical to the center line thereof. However, such Roberval mechanism is defective in that it has no possibility of acting as the ideal mechanism because of the fact that the links have integral construction to mutually provide adverse effect. In particular, in offset loading condition where the load is applied to any position offset from the center of the load cell, the strain-generating-portions 6a, 6b, 6c and 6d may be subjected to any moment caused by flexure of the strain-generating-body 2 that is in cantilevered form, thereby degrading the linearity between an amount of load and a detected output of the strain gauge.
Furthermore, an output circuit is formed by the Wheatstone bridge made up of the four strain gauges each adhered to each of four strain-generating-portions. Accordingly, if it is attempted to get the output from the strain gauges adhered to two strain-generating-portions of the arm at either one side, then an effect of moment caused by flexure of the strain-generating-body 2 in offset loading condition is more strongly reflected to the output, thereby further degrading the linearity.
In order to solve those problems the following techniques have been disclosed:
The first one is a load cell using a strain-generating-body comprising a Roberval mechanism, which is similar to the above-mentioned configuration, but is improved in that the thickness of strain-generating-portions 6a, 6b, 6c and 6d, or the volume constituting these portions is adjusted to control the rigidity of the strain-generating-portions 6a, 6b, 6c and 6d for deforming the strain-generating-body 2 as the ideal Roberval mechanism, thereby providing improved linearity. Such load cell can be found in Patent Document 1, for example, wherein a neutral axis of all the apertures 7, 8 and the channel 9 for forming the arms h1, h2 and the strain-generating-portions 6a, 6b, 6c and 6d is displaced from a neutral axis of the load cell itself so that there is difference in thickness between the upper and lower arms h1 and h2 and between the strain-generating-portions 6a, 6b, and 6c, 6d. 
The second one is a method of manufacturing a load cell, which can be found in Patent Document 2, for example, in which it comprises the steps of: adhering strain gauges 3a and 3b only on strain-generating-portions 6a and 6b, for example; making arms h1 and h2 to have same thickness and making all the strain-generating-portions 6a, 6b, 6c and 6d to have same thickness for improvement for linearity of the load cell when detecting any strain only at one side; deriving some relation equation expressing the relationship between the thickness of the strain-generating-portions 6a, 6b, 6c and 6d and that of the arms h1 and h2 in order to eliminate any effect of moment caused by flexure of the strain-generating-body 2 forming a cantilever construction; and manufacturing load cells having same shape according to the relation equation.
Patent Document 1: Japanese Patent Laid-Open No. 2000-214008
Patent Document 2: Japanese Patent No. 2666209
However, in case of the load cell, as disclosed in Japanese Patent Laid-Open No. 2000-214008, in which the thickness of the strain-generating-portions 6a, 6b, 6c and 6d is adjusted, or both thickness of the strain-generating-portions 6a, 6b, 6c and 6d and of the arms h1 and h2 are adjusted together, the present inventors have experimentally been found that, although some improved linearity may be attained for load directed to the twisted direction of the strain-generating-body 2 itself, as in the case of right and left offset loading relative to the measurement direction of the load cell, any span error due to offset loading still remains.
In addition, in case of the load cell, as disclosed in Japanese Patent No. 2666209, in order to eliminate any strain both in the strain-generating-portions and the arms due to flexure of the entire strain-generating-body 2, the upper and lower arms h1 and h2 and the strain-generating-portions 6a, 6b, 6c and 6d are made to have same thickness. Then, the thickness of both the upper and lower arms h1 and h2 are thinner relative to the thickness of the entire strain-generating-body 2, thereby leading to increase in flexure of the entire detector device. As the result, the natural frequency of the load cell becomes lower, and therefore, any oscillation in measurement environment is likely to be picked up to produce any disturbed output signal. Furthermore, because of longer period of time taken before load becomes stable the increased measurement time may be necessary. In addition, to keep the thickness of the arms, the strain-generating-body having larger size becomes necessary, which does not lend itself to provide lower profile construction and miniaturization.
In view of the above an object of the present invention is to solve the above-mentioned prior art problems by providing an improved load cell of Roberval type having strain gauges adhered to strain-generating-portions on one of two arms wherein the strain-generating-portions all have the same thickness and the two arms have different thickness in order to mitigate any effect of flexure of the strain-generating-body.